A conventional dielectric filter will be described below. FIG. 21 is a plan view showing the conventional dielectric filter. FIG. 22 is a sectional view taken along line A-A of FIG. 21.
The dielectric filter is constituted of first and second resonant elements 2 and 3 each of which is formed in the inner layer of a dielectric substrate 1 and has one open end and the other end connected to the ground, wide portions 2b and 3b which electromagnetically couple the first and second resonant elements 2 and 3 with each other and are formed on the sides of open ends 2a and 3a, narrow portions 2c and 3c formed on the side of a side electrode (a common ground terminal, i.e., the ground) 12 of the first and second resonant elements 2 and 3, bent portions 2d and 3d bent like letter L from the ends of the wide portions 2b and 3b to the side electrode 12, first and second input/output electrodes 4a and 5a formed in the upper layer of the bent portions 2d and 3d, input/output terminals 4b and 5b drawn from the input/output electrodes 4a and 5a, a capacitive electrode 11 formed in the upper layer of the wide portions 2b and 3b, an upper ground electrode 6 which is formed in the upper layer of the input/output electrodes 4a and 5a and connected to the side electrode 12, and a lower ground electrode 7 which is formed in the lower layer of the first and second resonant elements 2 and 3 and connected to the side electrode 12.
Ground patterns are formed over the upper ground electrode 6 and the lower ground electrode 7. A distance 8 between the electrode 6 and the wide portions 2b and 3b and a distance 8 between the electrode 6 and the narrow portions 2c and 3c are equal to each other. A distance 9 between the lower ground electrode 7 and the wide portions 2b and 3b and a distance 9 between the lower ground electrode 7 and the narrow portions 2c and 3c are equal to each other.
The input/output terminals 4b and 5b are drawn to the layer of the upper ground electrode 6 through inner vias 4c and 5c. Further, spaces 10a and 10b are provided between the input/output terminals 4b and 5b and the end face of the upper ground electrode 6. The spaces 10a and 10b of 150 μm or larger are necessary to prevent a short circuit on the input/output terminals 4b and 5b when the upper ground electrode 6 formed with a large pattern spreads during screen printing. The input/output terminals 4b and 5b are circular when viewed from the top. The input/output terminals 4b and 5b are about 200 μm in diameter. The dielectric filter protrudes by about 700 μm ((200 μm+150 μm)×2) in the lateral direction of FIG. 22 due to the presence of the input/output terminals 4b and 5b. 
As indicated by dotted lines in FIG. 21, the narrow portions 2c and 3c of the resonant elements 2 and 3 are arranged in parallel and electromagnetically coupled to each other. Further, the capacitive electrode 11 is electromagnetically coupled to the wide portions 2b and 3b. 
FIG. 23 is a replacement circuit diagram where the pattern of the dielectric filter is replaced with electric elements. In FIG. 23, reference numeral 4b denotes the input/output terminal and reference numeral 21 denotes a capacitance formed between the input/output electrode 4a and the wide portion 2b. Reference numeral 22 denotes an inductance of the narrow portion 2c and reference numeral 23 denotes a capacitance formed between the wide portion 2b and the ground electrodes 6 and 7. Similarly, reference numeral 24 denotes an inductance of the narrow portion 3c and reference numeral 25 denotes a capacitance formed between the wide portion 3b and the ground electrodes 6 and 7. Reference numeral 26 denotes a capacitance formed between the input/output electrode 5a and the wide portion 3b and reference numeral 5b denotes the input/output terminal connected to the capacitance 26. Reference numeral 27 denotes a capacitance between the wide portion 2b and the capacitive electrode 11 and reference numeral 28 denotes a capacitance between the wide portion 3b and the capacitive electrode 11. The inductances 22 and 24 are electromagnetically coupled to each other. Since the wide portions 2b and 3b are wide and short, the inductances thereof are negligible.
FIG. 24 is an equivalent circuit diagram of the replacement circuit diagram shown in FIG. 23. In FIG. 24, reference numeral 29 denotes a combined capacitance of the capacitance 27 and the capacitance 28 and reference numeral 30 denotes an inductance obtained by the electromagnetic coupling of the narrow portions 2c and 3c. The inductance 30 can be controlled by a distance 13 between the narrow portions 2c and 3c. In FIG. 24, the inductance 22 and the capacitance 23 are connected in parallel to form a parallel connection body 32. The parallel connection body 32 has one end connected to the input/output terminal 4b via the capacitance 21 and the other end connected to the ground.
Similarly, the inductance 24 and the capacitance 25 are connected in parallel to form a parallel connection body 33. The parallel connection body 33 has one end connected to the input/output terminal 5b via the capacitance 26 and the other end connected to the ground. A parallel connection body of the capacitance 29 and the inductance 30 is connected between one end of the parallel connection body 32 and one end of the parallel connection body 33, so that the parallel connection bodies entirely form a band-pass filter.
FIG. 25 is a signal pass characteristic diagram of the dielectric filter. A horizontal axis 34 represents a frequency, a vertical axis 35 represents an attenuation, and arrows represent directions that increase an attenuation. The pass band of the dielectric filter has a center frequency 36 proportionate to a factor of the square root of the product of the inductance 22 (or 24) and the capacitance 23 (or 25). According to the magnitude of the inductance 30 obtained by the electromagnetic coupling of the inductance 22 and the inductance 24, a narrow-band characteristic 37 or a wide-band characteristic 38 can be selected.
To be specific, the narrow-band characteristic 37 is obtained by increasing the distance 13 between the narrow portions 2c and 3c to have loose coupling or increasing the inductance 22 (or 24), and the wide-band characteristic 38 is obtained by reducing the distance 13 between the narrow portions 2c and 3c to have close coupling or reducing the inductance 22 (or 24). For example, Japanese Patent Laid-Open No. 7-142904 is known as prior art document information relating to the invention of this application.
In such a conventional dielectric filter, the upper ground electrode 6 is integrally formed over the upper layer of the input/output electrodes 4a and 5a, and the lower ground electrode 7 is integrally formed over the lower layer of the first and second resonant elements 2 and 3. That is, a distance 8 between the upper ground electrode 6 and the wide portions 2b and 3b and a distance 8 between the upper ground electrode 6 and the narrow portions 2c and 3c are equal to each other. The distance 9 between the lower ground electrode 7 and the wide portions 2b and 3b and the distance 9 between the lower ground electrode 7 and the narrow portions 2c and 3c are equal to each other.
When the areas of the wide portions 2b and 3b are reduced without changing the value of the capacitance 23 (or 25) to reduce the dielectric filter, it is necessary to reduce the distance 8 between the upper ground electrode 6 and the wide portions 2b and 3b and the distance 9 between the lower ground electrode 7 and the wide portions 2b and 3b. However, when the distance 8 between the upper ground electrode 6 and the narrow portions 2c and 3c or the distance 9 between the lower ground electrode 7 and the narrow portions 2c and 3c is reduced, Q of the inductance 22 (or 24) decreases, so that the loss of the pass band of the filter constituted of inductors, that is, an insertion loss increases and Q′ of frequency selectiveness decreases. Therefore, it is not possible to reduce the distance 8 or the distance 9 of the wide portions 2b and 3b and the narrow portions 2c and 3c which are integrally formed. Considering this restriction, it is not possible to reduce the dielectric filter without degrading its characteristics.
Further, since the upper ground electrode 6 has a relatively large pattern and conductive paste spreads during screen printing, it is necessary to make the spaces 10a and 10b larger than ordinary spaces, thereby increasing the protrusions of the input/output terminals 4b and 5b and the area of the filter.